1. Field of the Invention
This invention relates to processes for fabricating devices using photosensitive materials and in particular to processes employing a photosensitive material including an acid generator.
2. Art Background
Lithographic processes are typically employed in the manufacture of devices such as semiconductor devices. Among the lithographic processes that are available, photolithography is often utilized. Photolithographic processes have the advantage of being suitable for a blanket exposure technique. That is, a material that is sensitive to the exposing light is coated onto a substrate, e.g., a silicon wafer, that is being processed to fore a plurality of devices. The coating material, i.e., the resist, is then subjected to light that has been passed through a mask material so that the light reaching the resist corresponds to a desired pattern that is to be transferred into the underlying substrate. Since the exposure occurs simultaneously over an entire device or a number of devices being processed on a substrate, e.g., a silicon substrate, the procedure is considered a blanket exposure.
A blanket exposure procedure is advantageous because it is relatively fast compared to other methods such as the raster scan technique usually employed when the energy used to expose the resist is a beam of electrons. However, generally, resolution obtainable through a blanket exposure with ultraviolet or visible light is somewhat poorer than that achieved with other methods such as electron lithography.
One resist material, poly(methyl methacrylate) (PMMA) upon exposure with ultraviolet actinic radiation has shown resolution better than that achieved with exemplary resists used at conventional wavelengths (greater than 300 nm). For example, PMMA has been shown to be capable of resolution as good as about 250 nm. (See B. J. Lin, Journal of Vacuum Science and Technology, 12, 1317 (1975).) Although PMMA exhibits excellent resolution, its sensitivity to practical sources of actinic radiation is quite limited. Therefore, exposure times are generally excessive for practical applications.
Another suggested photoresist employs a compound that produces an acid moiety upon deep UV irradiation and a polymer that reacts in the presence of the generated acid to produce acidic substituents. Typical acid generator/acid sensitive polymer combinations include an onium salt e.g., triphenylsulfonium hexafluoroarsenate as the photosensitive acid generator and a polymer such as poly(p-t-butoxycarbonyloxystyrene) (PTBS) as the polymer having a reactive substituent.
Organic acid generators for use in acid generator/acid sensitive polymer combinations have been proposed. These acid generators include a variety of dinitro benzyl compounds as disclosed in U.S. Pat. No. 4,996,136 dated Feb. 26, 1991 which is hereby incorporated by reference. These acid generators have been shown to have good sensitivity to deep UV radiation. However, the materials are relatively costly and therefore acid generators with appropriate sensitivity and at decreased expense are very desirable.